The present invention relates to a multicolor image-forming material for forming a full color image of high definition with a laser beam, and a method for forming a multicolor image. In particular, the present invention relates to a multicolor image-forming material which is useful for forming a color proof (DDCP: direct digital color proof) or a mask image from digital image signals by laser recording in the field of printing, and a method for forming a multicolor image.
In the field of graphic arts, printing of a printing plate is performed by using a set of color separation films formed from a color original by lith films. In general, color proofs are formed from color separation films before real printing work for checking an error in the color separation step and the necessity for color correction. Color proofs are desired to realize high definition which makes it possible to surely reproduce a half tone image and have performances such as high stability of processing. Further, for obtaining color proofs closely approximating to an actual printed matter, it is preferred to use the materials which are used in actual printing as the materials for making color proofs, e.g., the actual printing paper as the substrate and the actual pigments as the coloring materials. As the method for forming color proofs, a dry method not using a developing solution is strongly desired.
As the dry method for forming color proofs, a recording system of directly forming color proofs from digital signals has been developed with the spread of electronized system in preprocessing of printing (pre-press field) in recent years. Such electronized system aims at forming in particular high quality color proofs, generally reproduces a dot image of 150 lines/inch or higher. For recording a proof of high image quality from digital signals, laser beams capable of modulation by digital signals and capable of finely diaphragming recording light are used as recording heads. Therefore, the development of an image-forming material having high recording sensitivity to laser beams and exhibiting high definition property capable of reproducing highly minute dots is required.
As the image-forming material for use in a transfer image-forming method using laser beams, a heat fusion transfer sheet comprising in the order of a support having a light-to-heat converting layer which absorbs laser beams and generates heat, and an image-forming layer which contains a pigment dispersed in components such as a heat fusion type wax and a binder is known (JP-A-5-58045 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)). In the image-forming method using such an image-forming material, an image-forming layer corresponding to the area of a light-to-heat converting layer irradiated with laser beams is fused by heat generated in that area and transferred onto an image-receiving sheet arranged on the transfer sheet by lamination, thus a transferred image is formed on the image-receiving sheet.
Further, a heat transfer sheet (thermal transfer sheet) comprising a support having provided thereon a light-to-heat converting layer containing a light-to-heat converting material, an extremely thin heat-releasing layer (from 0.03 to 0.3 xcexcm), and an image-forming layer containing a coloring material in this order is disclosed in JP-A-6-219052. In this heat transfer sheet, the bonding strength between the image-forming layer and the light-to-heat converting layer bonded through the intervening heat-releasing layer is reduced by laser beam irradiation, as a result, a highly minute image is formed on an image-receiving sheet arranged on the heat transfer sheet by lamination. The image-forming method by the heat transfer sheet utilizes so-called ablation, specifically the heat-releasing layer partially decomposes at the area irradiated with laser beams and vaporizes, thereby the bonding strength of the image-forming layer and the light-to-heat converting layer at that area is reduced and the image-forming layer at that area is transferred to the image-receiving sheet laminated thereon.
These image-forming methods have various advantages that an actual printing paper provided with an image-receiving layer (an adhesion layer) can be used as the material of an image-receiving sheet and a multicolor image can be easily obtained by transferring images different in colors in sequence on the image-receiving sheet, in particular, the image-forming method utilizing ablation has the advantage that highly minute image can be easily obtained, and so these methods are useful for forming a color proof (DDCP: direct digital color proof) or a highly minute mask image.
DTP is prevailing more and more and the intermediate process using films is omitted when CTP (computer to plate) is used, and the need for proof is shifting from analog proof to DDCP. In recent years the demand for large sized high grade DDCP highly stable and excellent in coincidence in printing has increased. High definition printing can be effected according to a heat transfer method by laser irradiation, and as the laser heat transfer methods, (1) a laser sublimation method, (2) a laser ablation method, and (3) a laser fusion method are conventionally used, but any of these methods has a drawback such that the shape of a recorded dots are not sharp. In (1) a laser sublimation method, since dyes are used as the coloring material, the approximation of proofs to printed matters is not sufficient, further, since this is a method of sublimating coloring materials, the outline of a dot is fuzzy, and so definition is not sufficiently high. On the other hand, since pigments are used as the coloring materials in (2) a laser ablation method, the approximation to printed matters is good, but since this is a method of sputtering coloring materials, the outline of a dot is also fuzzy as in the sublimation method, and so definition is not sufficiently high. Further, in (3) a laser fusion method, a molten substance flows, and so the outline of a dot is not also clear.
Accordingly, the subject of the present invention is to solve the above-described problems of the prior art techniques and to accomplish the following objects. That is, an object of the present invention is to provide a large sized high grade DDCP which is highly stable and excellent in coincidence in printing. Specifically, the present invention is characterized in that: 1) a heat transfer sheet is a membrane of coloring material which is not influenced by light sources of illumination as compared with pigment material and printed matters, and excellent in sharpness of dots and stability, 2) an image-receiving sheet can surely receive the image-forming layer in a heat transfer sheet by laser energy, 3) transfer to actual printing paper can be effected corresponding to the range of at least from 64 to 157 g/m2 such as art paper (coat paper), mat paper and finely coated paper, delicate texture can be imaged, and a high-key part can be reproduced accurately, and 4) extremely stable transfer releasability can be obtained. A further object of the present invention is to provide a method for forming a multicolor image which can form an image having good image quality and stable transfer image density on an image-receiving sheet even when recording is performed by multi-beam laser beams of high energy under different temperature conditions.
The present invention has been attained by the following means (1) to (46).
(1) A multicolor image-forming material comprising:
an image-receiving sheet having an image-receiving layer; and
at least four thermal transfer sheets each including a support, a light-to-heat converting layer and an image-forming layer, in which each of the thermal transfer sheets has a different color,
wherein an image is formed by: superposing the image-forming layer in each of the at least four thermal transfer sheets on the image-receiving layer in the image-receiving sheet, in which the image-forming layer is opposed to the image-receiving layer; irradiating the image-forming layer in each of the at least four thermal transfer sheets with a laser beam; and transferring the irradiated area of the image-forming layer onto the image-receiving layer in the image-receiving sheet, and
each of the light-to-heat converting layers in the at least four thermal transfer sheets has a ratio of an optical density (OD) to a layer thickness: OD/layer thickness (um unit) of 0.57 or more.
(2) The multicolor image-forming material as described in item (1), wherein the OD/layer thickness (um unit) is 1.95 or more.
(3) The multicolor image-forming material as described in item (1), wherein the OD/layer thickness (xcexcm unit) is 4.01 or more.
(4) The multicolor image-forming material as described in item (1), wherein the light-to-heat converting layer contains a light-to-heat converting material and the light-to-heat converting material is at least one of an organic dye and a carbon black, in which the organic dye is selected from the group consisting of a cyanine dye, an anthraquinone dye, an azulene dye and a phthalocyanine dye.
(5) The multicolor image-forming material as described in item (4), wherein the cyanine dye is an indolenine dye.
(6) The multicolor image-forming material as described in item (4), wherein the organic dye has a structure represented by formula: 
wherein R1 and R2, which are the same or different, each independently represents an alkyl group, an alkenyl group, or an aryl group; X represents an anion; and the naphthalene ring may be substituted or unsubstituted.
(7) The multicolor image-forming material as described in item (1), wherein the at least four thermal transfer sheets have four or more colors including yellow, magenta, cyan and black, and each of the at least four thermal transfer sheets has a different color, and each of the light-to-heat converting layers in the at least four thermal transfer sheets has a different optical density per the light-to-heat converting layer, in which each of the light-to-heat converting layers in the at least four thermal transfer sheets each having a different color has the same optical density per unit thickness of the light-to-heat converting layer and has a different layer thickness.
(8) The multicolor image-forming material as described in item (7), wherein each of the light-to-heat converting layers in the at least four thermal transfer sheets each having a different color is formed with the same coating solution.
(9) The multicolor image-forming material as described in item (7), wherein the light-to-heat converting layer in the black thermal transfer sheet has the lowest optical density in the at least four thermal transfer sheets.
(10) The multicolor image-forming material as described in item (7), wherein the image-receiving sheet includes a support, and each of the light-to-heat converting layers in the at least four thermal transfer sheets each having a different color contains a matting agent in a different amount.
(11) The multicolor image-forming material as described in item (10), wherein each of the light-to-heat converting layers in the at least four thermal transfer sheets each having a different color has the same content of the matting agent per unit thickness of the light-to-heat converting layer and has a different layer thickness.
(12) The multicolor image-forming material as described in item (10), wherein each of the light-to-heat converting layers in the at least four thermal transfer sheets each having a different color is formed with the same coating solution.
(13) The multicolor image-forming material as described in item (10), wherein the light-to-heat converting layer in the black thermal transfer sheet has the lowest content of the matting agent in the at least four thermal transfer sheets.
(14) The multicolor image-forming material as described in item (7), wherein the light-to-heat converting layer has an optical density of from 0.80to 1.49at peak wavelength of the laser beam.
(15) The multicolor image-forming material as described in item (7), wherein the light-to-heat converting layer contains a light-to-heat converting agent and the light-to-heat converting agent is a compound other than carbon black, graphite and colloidal silver.
(16) The multicolor image-forming material as described in item (15), wherein the light-to-heat converting agent is a cyanine dye.
(17) The multicolor image-forming material as described in item (1), wherein the light-to-heat converting layer contains a light-to-heat converting material and a resin, and the half value width of the maximum absorbance at wavelength of 700 to 1,200 nm of the light-to-heat converting layer is 200 nm or less.
(18) The multicolor image-forming material as described in item (17), wherein the thermal transfer sheet includes an intermediate layer between the light-to-heat converting layer and the image-forming layer.
(19) The multicolor image-forming material as described in item (17), wherein the light-to-heat converting material is an infrared absorbing dye.
(20) The multicolor image-forming material as described in item (19), wherein the infrared absorbing dye is a cyanine dye.
(21) The multicolor image-forming material as described in item (17), wherein irradiation is performed with a laser having an output of 50 mW or more at linear velocity of 7 m/s or more.
(22) The multicolor image-forming material as described in item (1), wherein each of the light-to-heat converting layer and the image-forming layer contains a water-insoluble resin and the recorded image has a resolution of 2,400 dpi or more.
(23) The multicolor image-forming material as described in item (22), wherein the resin in the light-to-heat converting layer is soluble in a hydrophobic solvent having an SP value of from 16 to 22.
(24) The multicolor image-forming material as described in item (22), wherein the resin in the image-forming layer is soluble in a nonaqueous solvent having an SP value of from 16 to 30.
(25) The multicolor image-forming material as described in item (22), wherein a hydrophobic solvent is used for dissolving the resin when the light-to-heat converting layer is provided, and a nonaqueous solvent is used for solving the resin when the image-forming layer is provided.
(26) The multicolor image-forming material as described in item (22), wherein the light-to-heat converting material in the light-to-heat converting layer has the solubility of 1 weight % or less in a coating solvent for forming the image-forming layer.
(27) The multicolor image-forming material as described in item (22), wherein the resin in the light-to-heat converting layer has the solubility of 1 weight % or less in a coating solvent for forming the image-forming layer.
(28) The multicolor image-forming material as described in item (22), wherein the resin in the light-to-heat converting layer has the solubility of 0.1 weight % or more in the coating solvent for forming the light-to-heat converting layer.
(29) The multicolor image-forming material as described in item (22), wherein the light-to-heat converting material has the solubility of 0.1 weight % or more in the coating solvent for forming the light-to-heat converting layer.
(30) The multicolor image-forming material as described in item (22), wherein the light-to-heat converting material is an infrared absorbing dye.
(31) The multicolor image-forming material as described in item (22), wherein the resin of the image-forming layer has the solubility of 0.1 weight % or more in the coating solvent for forming the image-forming layer.
(32) The multicolor image-forming material as described in item (22), wherein the thermal transfer sheet includes an intermediate layer between the light-to-heat converting layer and the image-forming layer.
(33) The multicolor image-forming material as described in item (1), wherein the transferred image has a resolution of 2,400 dpi or more.
(34) The multicolor image-forming material as described in item (33), wherein the transferred image has a resolution of 2,600 dpi or more.
(35) The multicolor image-forming material as described in item (1), wherein the recording area of the multicolor image is a size of 515 mm or more multiplying 728 mm or more.
(36) The multicolor image-forming material as described in item (35), wherein the recording area of the multicolor image is a size of 594 mm or more multiplying 841 mm or more.
(37) The multicolor image-forming material as described in item (1), wherein each of the image-forming layers in the at least four thermal transfer sheets has a ratio of an optical density (OD) to a layer thickness: OD/layer thickness (xcexcm unit) of 1.50 or more.
(38) The multicolor image-forming material as described in item (37), wherein the OD/layer thickness (xcexcm unit) is 1.80 or more.
(39) The multicolor image-forming material as described in item (38), wherein the OD/layer thickness (um unit) is 2.50 or more.
(40) The multicolor image-forming material as described in item (1), wherein the image-forming layer in each of the at least four thermal transfer sheets and the image-receiving layer in the image-receiving sheet each has a contact angle with water of from 7.0 to 120.0xc2x0.
(41) The multicolor image-forming material as described in item (1), wherein the ratio of an optical density (OD) of the image-forming layer in each of the at least four thermal transfer sheets to a thickness of the image-forming layer: OD/layer thickness (xcexcm unit) is 1.80 or more and the image-receiving layer in the image-receiving sheet has a contact angle with water of 86xc2x0 or less.
(42) A method for manufacturing the multicolor image-forming material as described in item (1), which comprises performing a successive coating.
(43) A method for forming a multicolor image, which comprises the steps of:
preparing: an image-receiving sheet having an image-receiving layer; and at least four thermal transfer sheets each including a support, a light-to-heat converting layer and an image-forming layer, in which each of the at least four thermal transfer sheets has a different color and each of the light-to-heat converting layers in the at least four thermal transfer sheets has a ratio of an optical density (OD) to a layer thickness: OD/layer thickness (xcexcm unit) of 0.57 or more;
superposing the image-forming layer in each of the at least four thermal transfer sheets on the image-receiving layer in the image-receiving sheet, in which the image-forming layer is opposed to the image-receiving layer;
irradiating the image-forming layer in each of the at least four thermal transfer sheets from the side of the support with a laser beam; and
transferring the irradiated area of the image-forming layer onto the image-receiving layer in the image-receiving sheet to record an image,
wherein the irradiated area of the image-forming layer is transferred onto the image-receiving sheet in a thin film.
(44) The method for forming a multicolor image as described in item (43), wherein the laser beam is a semiconductor laser beam.
(45) The method for forming a multicolor image as described in item (43), wherein the laser beam comprises multi-beams of two-dimensional array.
(46) A method for preparing a color proof, which comprises:
preparing: an image-receiving sheet having an image-receiving layer; and at least four thermal transfer sheets each including a support, a light-to-heat converting layer and an image-forming layer, in which each of the at least four thermal transfer sheets has a different color and each of the light-to-heat converting layers in the at least four thermal transfer sheets has a ratio of an optical density (OD) to a layer thickness: OD/layer thickness (xcexcm unit) of 0.57 or more;
superposing the image-forming layer in each of the at least four thermal transfer sheets on the image-receiving layer in the image-receiving sheet, in which the image-forming layer is opposed to the image-receiving layer;
irradiating the image-forming layer in each of the at least four thermal transfer sheets from the side of the support with a laser beam;
transferring the irradiated area of the image-forming layer onto the image-receiving layer in the image-receiving sheet to record a full color image, wherein the irradiated area of the image-forming layer is transferred onto the image-receiving sheet in a thin film; and
transferring the full color image on the image-receiving layer onto an actual printing paper.